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United States Patent |
5,275,044
|
Riley
|
January 4, 1994
|
Three wire potentiometric liquid level sensor
Abstract
A three wire potentiometric liquid level sensor for measuring liquid levels
in a container is disclosed. The sensor includes a conductive strip, a
float, a contact means attached to the float, a resistive strip, and a
conductive rod buried beneath an insulator. A second embodiment of the
invention includes a first conductive strip, a second conductive strip, a
float, a contact means attached to the float, and an insulating rod. The
sensor produces a resistance directly proportional to the liquid level in
the container.
Inventors:
|
Riley; Richard E. (Riverside, CA)
|
Assignee:
|
Spectrol Electronics Corporation (Ontario, CA)
|
Appl. No.:
|
876575 |
Filed:
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April 30, 1992 |
Current U.S. Class: |
73/313; 73/319; 338/33 |
Intern'l Class: |
G01F 023/60 |
Field of Search: |
73/313,304 R,308,DIG. 5,319
338/33
|
References Cited
U.S. Patent Documents
4052901 | Oct., 1977 | Bjork | 338/33.
|
4345235 | Aug., 1982 | Riley et al. | 338/176.
|
4702107 | Oct., 1987 | Guerrini | 338/33.
|
4724705 | Feb., 1988 | Harris | 338/33.
|
4827769 | May., 1989 | Riley et al. | 73/313.
|
4920798 | May., 1990 | Weaver | 73/319.
|
5020366 | Jun., 1991 | Elfverson et al. | 73/313.
|
5129261 | Jul., 1992 | Riley | 73/308.
|
5138881 | Aug., 1992 | Riley et al. | 338/33.
|
5146785 | Sep., 1992 | Riley | 338/33.
|
Primary Examiner: Yasich; Daniel M.
Attorney, Agent or Firm: Woodard, Emhardt, Naughton Moriarty & McNett
Parent Case Text
This application is a division of application Ser. No. 07/658,103, filed
Feb. 20, 1991, which issued Jul. 14, 1992, as U.S. Pat. No. 5,129,261.
Claims
What is claimed is:
1. A liquid level sensor for providing a variable resistance corresponding
to fluid level in a container, said sensor comprising:
a non-conductive rod having a first end and a second end, said rod having
at least one longitudinal planar surface;
a resistive strip attached to said rod, said resistive strip substantially
axially aligned with said rod;
a first conductive strip attached to said rod, said conductive strip
disposed substantially in parallel with said resistive strip, said first
conductive strip electrically connected to said resistive strip near said
second end of said rod;
a second conductive strip attached to said rod, said second conductive
strip disposed substantially in parallel with said resistive strip;
a float having a hole for axially receiving said rod therein, wherein said
float is disposed over said rod and positioned along said rod according to
the liquid level in the container;
electrical contact means attached to said float for continuously
establishing an electrical connection between said resistive strip and
said second conductive strip:
wherein said electrical contact means includes at least two metallic
contacts situated in substantially opposing relationship and spring biased
toward one another, said contacts receiving said rod therebetween, and
wherein said resistive strip and said second conductive strip are disposed
on opposite sides of said rod so that said metallic contacts make contact
with and electrically connect said resistive strip and said second
conductive strip; and
wherein a DC voltage is applied between said resistive strip and said first
conductive strip, so that the location of said float with respect to said
rod is determined by comparing the voltage appearing on said second
conductive strip with said DC voltage.
2. The liquid level sensor of claim 1 including a plurality of uniformly
spaced conductor bars attached to said resistive strip and wherein said
contact means establishes an electrical connection between said conductor
bars and said conductive strip.
3. The liquid level sensor of claim 1 wherein said resistive strip is a
screen printed cermet thick film and said first and second conductive
strips are a screen printed conductive thick film, and wherein said
resistive strip and said first and second conductive strips are thermally
fused to said non-conductive rod.
4. The liquid level sensor of claim 3 including a plurality of conductor
bars attached to said resistive strip and wherein said electrical contact
means establishes an electrical connection between said conductor bars and
said second conductive strip.
5. The liquid level sensor of claim 4 including connection means attached
to said rod for enabling an electrical connection from said first
conductor means, said second conductor means and from said resistance
means to an external resistance or voltage measuring device.
6. The sensor of claim 5 wherein said connection means includes:
a first terminal attached to said first conductive strip;
a second terminal attached to said resistive strip; and
a third terminal attached to said second conductive strip.
7. A liquid level sensor comprising:
an elongated non-conductive member having a substantially vertically
oriented axis and including a longitudinal planar surface;
resistance means attached to said member and substantially axially aligned
with said member;
first conductor means attached to said member and positioned substantially
in parallel with said resistance means, said first conductor means
electrically connected to said resistance means at the lower end of said
resistance means;
second conductor means attached to said member and positioned substantially
in parallel with said resistance means;
a float having a hole therethrough and disposed about said member, said
float being positionable along said non-conductive member in accordance
with liquid level; and
electrical contact means attached to said float for establishing an
electrical connection between said resistance means and said second
conductor means;
wherein said resistance means is a screen printed cermet thick film and
said first and second conductor means are screen printed conductive thick
film, and wherein said resistive means, said first conductor means and
said second conductor means are thermally fused to said insulator; and
wherein a DC voltage is applied between said resistance means and said
first conductor means so that the location of said float with respect to
said non-conductive member is determined by comparing the voltage
appearing on said second conductor means with said DC voltage.
8. The liquid level sensor of claim 7 wherein said electrical contact means
includes at least two metallic contacts situated in substantially opposing
relationship and spring biased toward one another, said contacts receiving
said non-conductive member therebetween.
9. The liquid level sensor of claim 8 including a plurality of uniformly
spaced conductor bars attached to said resistance means and wherein said
electrical contact means establishes an electrical connection between said
conductor bars and said conductor means.
10. The liquid level sensor of claim 9 wherein said elongated
non-conductive member is a rod having a rectangular cross-section.
11. The liquid level sensor of claim 10 including
a first conductive terminal attached to said first conductive strip;
a second conductive terminal attached to said resistive strip; and
a third conductive terminal attached to said second conductive strip.
Description
FIELD OF THE INVENTION
This invention relates to devices for detecting liquid level within a
container, and more specifically to sensors for sensing the liquid level
of fuel in a motor vehicle fuel tank.
BACKGROUND OF THE INVENTION
The most commonly used fluid level sensor is the variable resistor sensor
utilizing a float to produce a resistance change in the variable resistor.
As the float moves vertically with the fluid level, the electrical
resistance of the sensor changes typically from 10 to 400 ohms. In most
sensors, a sliding or moving contact attached to the float establishes a
resistive circuit based upon the position of the contact with respect to a
wirewound resistor or a thick film resistor printed on an insulating base
or substrate.
Other approaches to fluid level detection include the use of resistors with
large temperature coefficients, known as thermistors, located at various
vertical positions in the fluid reservoir. As electrical power is applied
to the resistors, the devices immersed in the fluid remain cool while
those that are exposed to air will increase in temperature and produce a
change in overall resistance of the device. Extensive signal conditioning
and temperature compensation circuitry is typically required with such a
sensor to create a usable signal. Fluid compatibility and manufacturing
costs limit widespread acceptance of this type of device.
A vertical sensor with a sliding contact has been used in some automotive
applications. Typically a float provides a contact point with respect to a
resistor. The resistor is usually a wire helix wound about an insulating
mandrel.
Examples of prior art fliud level sensors are shown in the following
patents: Weaver, U.S. Pat. No. 4,920,798, Riley et al., U.S. Pat. No.
4,827,769, Guerrini et al., U.S. Pat. No. 4,702,107, Hoppert et al., U.S.
Pat. No. 4,567,762, Coulange, U.S. Pat. No. 4,454,761, Bjork, U.S. Pat.
No. 4,052,901, DeGiers, U.S. Pat. No. 2,484,690, and German Patent
2758379.
An example of thick film resistor technology used in a liquid level sensor
is shown in Weaver, U.S. Pat. No. 4,920,798. The Weaver device includes a
thick film resistive coated plate with a slidable contact member providing
a resistance in proportion to the position of a float mounted on an
adjacent rod.
Riley et al., U.S. Pat. No. 4,827,769, discloses a fuel level sensor
including a soft steel substrate encased in porcelain with a cermet thick
film resistive track deposited thereon. The Riley device provides a
resistance proportional to float position.
Guerrini et al., U.S. Pat. No. 4,702,107, discloses a device for detecting
the level of a liquid contained in a tank and includes a vertical bar and
a float positioned according to liquid level. The Guerrini device
establishes an inverse correlation between circuit resistance and fluid
level.
Hoppert et al. U.S. Pat. No. 4,567,762 discloses a thermoelectric level
detector including a meandering resistive path which repeatedly
transverses the axis of the impedance element. The resistance of the
meandering path is dependent upon temperature and provides a measure of
liquid level.
Coulange, U.S. Pat. No. 4,454,761 discloses a liquid level detector
including a slidable float and a winding disposed about the periphery of a
rod. The float's vertical position, as defined by the liquid level,
controls the resistance of the winding.
Bjork, U.S. Pat. No. 4,052,901 discloses a detecting apparatus including an
elongated flexible substrate transducer which is shorted out in the
portion thereof subject to a threshold pressure. The resistance produced
is inversely proportional to the liquid level.
DeGiers, U.S. Pat. No. 2,484,690 discloses an electric liquid level
indicating device including a resistor element, a common conductor, a
plurality of flexible sliding members between the resistor element and the
common conductor, and a magnet attached to a float. The magnet shorts out
the resistor at the liquid level by attracting a corresponding flexible
sliding member.
German Patent 2758379 discloses a potentiometer for indicating liquid
levels including a chain of resistors connected to reed relays. The relays
are actuated by a permanent magnet attached to a float. The resistance of
the potentiometer is proportional to the liquid level.
An improved liquid level sensor with highly reliable components yet
economical to manufacture is needed.
SUMMARY OF THE INVENTION
A liquid level sensor for providing a variable resistance corresponding to
fluid level in a container according to one aspect of the present
invention comprises a conductive rod having a first end and a second end.
An insulator is attached to and covers the conductive rod. The insulator
includes a first aperture near the second end of the conductive rod. A
resistive strip is attached to the insulator. The resistive strip is
substantially aligned with the longitudinal axis of the conductive rod,
the resistive strip being electrically connected to the rod through the
first aperture. A conductive strip is attached to the insulator and
disposed substantially in parallel with the resistive strip. A float
having a hole axially receives the insulator covered conductive rod
therein, wherein the float is positioned relative to the rod according to
the liquid level in the container. Electrical contact means are attached
to the float for establishing an electrical connection between the
resistive strip and the conductive strip.
A liquid level sensor for providing a variable resistance corresponding to
fluid level in a container according to another aspect of the present
invention includes a non-conductive rod having a first end and a second
end. A resistive strip is attached to the rod, the resistive strip
substantially axially aligned with the rod. A first conductive strip is
attached to the rod, the conductive strip disposed substantially in
parallel with the resistive strip. The first conductive strip is
electrically connected to the resistive strip near the second end of the
rod. A second conductive strip is attached to the insulator, the second
conductive strip being disposed substantially in parallel with the
resistive strip. A float having a hole axially receives the rod therein,
wherein the float is positioned relative to the rod according to the
liquid level in the container. Electrical contact means are attached to
the float for continuously establishing an electrical connection between
the resistive strip and the second conductive strip.
One object of the present invention is to provide an improved liquid level
sensor.
A second object of the present invention is to provide a liquid level
sensor which is configurable to provide a resistance proportional to the
cross-sectional contours of the container in which the sensor is mounted.
A third object of the present invention is to provide a reliable liquid
level sensor with a resistive element of improved reliability.
Related objects and advantages of the present invention will be more
apparent from the following description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front elevational view of a first embodiment of a liquid level
sensor according to the present invention.
FIG. 1 is a plan view of the sensor of FIG. 1.
FIG. 1B is an enlarged cross-sectional view of the sensor of FIG. 1 looking
in the direction of the arrows labelled 1B.
FIG. 2 is a front elevational view of a second embodiment of a liquid level
sensor according to the present invention.
FIG. 2A is a plan view of the sensor of FIG. 2.
FIG. 2B is an enlarged cross-sectional view of the sensor of FIG. 2 looking
in the direction of the arrows labelled 2B.
FIG. 3 is a rear elevational view of the sensor of FIG. 1.
FIG. 3A is a rear elevational view of the sensor of FIG. 2.
FIG. 4 is a partial cross-sectional side view depicting the contacts within
the float of the sensor in FIG. 1.
FIG. 4A is a partial cross-sectional side view depicting the contacts
within the float of the sensor in FIG. 2.
FIG. 5 is a cross-sectional view looking in the direction of the arrows
labelled 5 in FIG. 1.
FIG. 5A is a cross-sectional view looking in the direction of the arrows
labelled 5A in FIG. 2.
FIG. 6 is an enlarged partial front elevational view of the resistive strip
and conductor bars of the sensor of FIG. 1.
FIG. 6A is an enlarged partial front elevational view of the resistive
strip and conductor bars of the sensor of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of promoting an understanding of the principles of the
invention, reference will now be made to the embodiment illustrated in the
drawings and specific language will be used to describe the same. It will
nevertheless be understood that no limitation of the scope of the
invention is thereby intended, such alterations and further modifications
in the illustrated device, and such further applications of the principles
of the invention as illustrated therein being contemplated as would
normally occur to one skilled in the art to which the invention relates.
Referring now to FIGS. 1, 1A, and 1B, a first embodiment of a liquid level
sensor 10 according to the present invention is illustrated. Sensor 10
includes an insulated conductive strip or rod 12 covered by a thin
insulator 20. Rod 12 functions as a "buried conductor", providing a
conductive path beneath insulator 20. A resistive strip 22 is attached to
insulator 20 and is aligned with the longitudinal axis 18 of rod 12.
Insulator 20 includes an aperture at 16 through which an electrical
connection between the rod 12 and resistive strip 22 is established.
Parallel alignment of the resistive strip 22 with the axis 18 is not a
necessity for the sensor 10 to function properly, so long as the contacts
40 and 42 (shown in FIG. 4) make electrical contact with conductor bars
26.
A plurality of electrically isolated uniformly spaced conductor bars 26 are
attached to resistive strip 22. Mechanical wiping of contact 40 (see FIG.
4) on conductor bars 26 prolongs the useful life of the sensor 10 since
the resistive strip 22 is unaffected by repeated contact between the bars
26 and contacts 40 and allows up to 100 milliamps to be applied through
the wiper. Lower contact resistance allows more current to pass through
contact 40 to resistive strip 22 without oxidation or erosion of the
conductive strip 32 and contact 42.
Probe pads 33 are attached to insulator 20 and electrically connected to
resistive strip 22. Probe pads 33 enable electrical contact with
predetermined locations on resistive strip 22 for laser trimming purposes.
Laser trimming of the resistive strip 22 is one technique known for
tailoring the resistance of resistive strip 22 to the cross-sectional
contour of the liquid container in which the sensor is utilized. However,
where the tank cross-section is rectangular, resistive strip 22 will also
be rectangular.
Rod 12 has a rectangular cross-section (see FIG. 1B) and is preferably made
of steel. Insulator 20 prevents galvanic activity which may cause erosion
of resistive strip 22 and conductive strip 32 (see FIG. 3) when the strips
22 and 32 are subjected to DC power in a conductive media such as water.
Water often collects in the bottom of fuel tanks. Such electrolytic action
is prevented by depositing an insulator 20, a low alkali porcelain enamel
coating with a thickness of 0.2 mm, on rod 12. Resistive strip 22 is a
cermet (ceramic and metallic) thick film fabricated using glass and metal
particles. The cermet material consists of approximately 80% glass fused
to the porcelain enamel of insulator 20 in a 0.025 mm thick layer. The
high glass content prevents oxidation or chemical attack on the metallic
components of resistive strip 22. The composition of resistive strip 22
provides a resistance per unit length in the range of 10 to 100 ohms per
linear inch. Resistive strip 22 is attached to insulator 20 by screen
printing, sputtering, electrochemical etching, or other applicable method
known in the art of thick film processing.
Rod 12 is electrically connected to resistive strip 22 through an aperture
in insulator 20 at 16, hereinafter referred to as the interface between
resistive strip 22 and rod 12. The interface includes a nickel
metallization layer which is thermally bonded to steel rod 12 in the
aperture at 16. A palladium and silver thick film material is next
thermally bonded to the nickel metallization layer. The bonding agent used
to thermally bond the palladium/silver thick film to the nickel
metallization layer is glass. Resistive strip 22 is applied over the
bonding agent and thermally fused thereto. The interface is one suggested
technique to electrically connect rod 12 to resistive strip 22.
Float 36, shown in cutaway form, is disposed about the periphery of the
insulator 20 and rod 12. Float 36 is typically made from a closed cell
foam material well known in the art. One closed cell foam material is
known commonly as nitrile rubber. Float 36 is positioned vertically
according to the liquid level in the container.
Terminal 44 is electrically and mechanically attached to insulated rod 12
with fastener or rivet 24 through a hole in rod 12 at 14. Fastener 24 is a
rivet, however, solder is also contemplated for attaching the terminals to
the sensor 10. An opening or aperture in insulator 20 at 12a facilitates
an electrical connection between the terminal 44 and rod 12. A nickel
metallization layer is applied to the exposed rod 12 at 12a to prevent
corrosion of rod 12 where exposed. Terminal 44 includes barbs or raised
abrasive points disposed toward the mating surface between terminal 44 and
rod 12 to facilitate a good electrical connection to the rod 12. Terminal
46 is electrically connected to resistive strip 22 by thin film conductive
strip 21. Terminal 46 is secured to the rod/insulator by a fastener or
rivet 25 which extends through an aperture or hole in rod 12 at 15. The
aperture at 15 is entirely coated on the interior by insulator 20.
Non-conductive centering washers 23 (made of non-conductive material such
as nylon or other suitable polymers) isolate terminal 48 from rod 12 as
well as from terminals 44 and 46. Terminals 44 and 46 are also made from
tin plated brass. Conductive strip 21 is a thick film material consisting
of glass, palladium and silver which is thermally fused to insulator 20.
Referring now to FIG. 4, a cross-sectional view of the float of the sensor
10 of FIG. 1 is shown. Rod 12 is axially inserted through hole 38 in float
36. Contact 40 and contact 42 are spring biased towards rod 12, and
comprise an electrical contact means which provides an electrical
connection between conductor bars 26, contact 42, contact 40, and
conductive strip 32. Resistive strip 22 is also shown.
A preferred design of contact 40 and contact 42 is a multi-finger hoe
configuration (shown in FIG. 5). This configuration adds to the
reliability of the sensor because it applies low frictional force against
conductor bars 26 and conductive strip 32. Electrical noise is minimized
with the contact configuration shown. The configuration of contacts 40 and
42 also provides high corrosion resistance and a constant cross-sectional
area over the life of the sensor. Contacts 40 and 42 are attached to float
36 and extend into hole 38 to minimize any physical damage to the contacts
during manufacturing. Contacts 40 and 42 are electrically connected.
Contact tips 40a and 42a are attached to contacts 40 and 42, respectively.
Contact tips 40a and 42a are made of materials suitable for electrical
contact applications.
A preferred precious metal alloy used in the construction of contact tips
40a and 42a is palladium and silver or solid silver contacts. Typically
the contact tips 40a and 42a are precious metal and the remaining portion
of the contacts 40 and 42 is beryllium copper. Beryllium copper, as is
well-known in the art, is a resilient spring-like material which is often
used in leaf spring contact applications. Other precious metal alloys such
as silver/nickel and silver/cadmium alloys are also contemplated as
materials which may be used for fabricating contact tips 40a and 42a. The
precious metal contact tips are joined to the beryllium copper leaf spring
by rivets, crimping, silver soldering or other well-known attachment
techniques.
Referring to FIG. 5, cross-sectional view looking in the direction of the
arrows labelled 5 of the embodiment of FIG. 1 is shown. Rod 12 is shown
located beneath the insulator 20. Float 36 is disposed about the rod and
insulator. Hole 38 is sized to receive and center rod 12 with respect to
contact tips 40a and 42a yet allow free movement of float 36 vertically
with respect to rod 12. Contact tips 40a and 42a make physical contact
with conductor bars 26 and conductive strip 32, respectively. Resistive
strip 22 is shown attached to insulator 20 and conductor bars 26.
FIG. 6 is a partial front elevational view of the resistive strip 22 and
conductor bars 26 of the sensor 10. Conductor bars 26 extend from
resistive strip 22 at an angle slightly different from the perpendicular
to prevent contact tip 40a from falling in the non-conductive porcelain
area between each of the bars 26. Conductor bars 26 are uniformly spaced
and arranged in parallel to provide electrical contact with resistive
strip 22.
Referring now to FIGS. 2, 2A and 2B, a second embodiment of a sensor 110
according to the present invention is illustrated. Sensor 110 includes a
non-conductive rod 112 to which resistive strip 122, conductive strip 121
and conductive strip 131 are attached. Strips 121, 122 and 131 are thick
film materials thermally bonded to the surface of the rod 112.
Conductive strips 121 and 131 overlap strip 122 at 116 and 116a,
respectively to make electrical connections with the ends of resistive
strip 122. Thermal bonding of the thick film materials of strips 122 and
131 establishes the electrical connection between the strips. The
conductive strips 121 and 131 are thick film materials consisting of
glass, palladium and silver which are thermally used to rod 112. Terminal
144 is electrically connected with conductive strip 131 beneath terminal
144. Similarly, terminal 146 is electrically connected with conductive
strip 121 beneath terminal 146. It is also contemplated that terminals 144
and 146 may be soldered to conductive strips 131 and 121, respectively, to
complete the corresponding electrical connections. Terminals 144 and 146
may include barbs (not shown) or raised projections facing inward toward
rod 112. The barbs aid in establishing an electrical connection with the
strips 121 and 131 when mechanical fasteners such as rivets 124 and 125
are used to attach the terminals 144, 146 and 148 to the rod 112. Terminal
144 and terminal 146 are attached to rod 112 through holes at 114 and 115,
respectively. Insulating washers 123 isolate terminals 146 and 148 from
terminal 148 and perform the same function as washers 23 of sensor 10.
Float 136 moves along axis 118 in accordance with liquid levels in the
container in which sensor 110 is situated. Probe pads 133 provide
convenient manufacturing connections to predetermined locations of
resistive strip 122 to assist in laser trimming operations wherein the
resistance of the strip 122 is "trimmed" to correspond with the
cross-sectional contours of the container (not shown) in which the sensor
110 will be used. Conductor bars 126 are identical with the conductor bars
26 of the embodiment of FIG. 1, and provide a low contact resistance
connection between the resistive strip 122 and the float contacts 140 and
142 (shown in FIG. 4A).
Non-conductive rod 112 is a ceramic substrate having a rectangular
cross-section. An alumina ceramic substrate is preferred for the rod 112.
Other non-conductor materials are also contemplated as suitable substrate
materials.
Referring now to FIG. 3, a rear elevational view of the sensor 10 of FIG. 1
is illustrated. Conductive strip 32, fabricated using thick film
technology, is attached to insulator 20. Strip 32 consists of a mixture of
glass, palladium, and silver which presents a smooth, non-abrasive surface
against which contact 40 (FIG. 4) brushes. Conductive strip 32 is made
from the same material as conductive strips 21 and 31. Conductive strips
21, 31 and 32 fuse with and slightly sink into the porcelain enamel of
insulator 20 or rod 13 during thermal bonding. Conductive strips 21, 31
and 32 and resistive strip 22 are deposited by screen printing,
sputtering, electrochemical etching, or any other applicable method.
Terminal 48 is electrically attached to conductive strip 32 adjacent
terminal 44 and terminal 46. Terminal 48 includes barbs to ensure proper
electrical connections and is made from tin plated brass.
Referring now to FIG. 3A, a rear elevational view of the sensor 110 of FIG.
2 is illustrated. Conductive strip 132, fabricated using thick film
technology, is attached to rod 112. Strip 132 consists of a mixture of
glass, palladium, and silver which presents a smooth, non-abrasive surface
against which contact 140 (FIG. 4A) brushes. Conductive strip 132 is made
from the same material as conductive strips 121 and 131. Conductive strips
121, 131 and 132 and resistive strip 122 are deposited by screen printing,
sputtering, electrochemical etching, or any other applicable method and
subsequently thermally fused to the substrate or rod 112. Terminal 148 is
electrically attached to conductive strip 132 and is located opposite
terminal 144 and terminal 146. Terminal 148 includes barbs to ensure
proper electrical connections and is made from tin plated brass.
Referring now to FIG. 4A, a cross-sectional view of the float 136 of sensor
110 is shown. Rod 112 is axially inserted through hole 138 in float 136.
Contact 140 and contact 142 are spring biased towards rod 112, and
comprise an electrical contact means which provides an electrical
connection between conductor bars 126 and conductive strip 132. Resistive
strip 122 is also shown.
A preferred design of contact 140 and contact 142 is a multi-finger hoe
configuration (shown in FIG. 5A). This configuration adds to the
reliability of the sensor because it applies low frictional force against
conductor bars 126 and conductive strip 132. Electrical noise is minimized
with the contact configuration shown. The configuration of contacts 140
and 142 also provides high corrosion resistance and a constant
cross-sectional area over the life of the sensor. Contacts 140 and 142 are
attached to float 136 and extend into hole 138 to minimize any physical
damage to the contacts during manufacturing. Contacts 140 and 142 are
electrically connected. Contact tips 140a and 142a are attached to
contacts 140 and 142, respectively. Contact tips 140a and 142a are made of
materials suitable for electrical contact applications.
A preferred precious metal alloy used in the construction of contact tips
140a and 142a is palladium and silver or solid silver contacts. Typically
the contact tips 140a and 142a are precious metal and the remaining
portion of the contacts 140 and 142 is beryllium copper. Beryllium copper,
as is well-known in the art, is a resilient spring-like material which is
often used in leaf spring contact applications. Other precious metal
alloys such as silver/nickel and silver/cadmium alloys are also
contemplated as materials which may be used for fabricating contact tips
140a and 142a. The precious metal contact tips are joined to the beryllium
copper leaf spring by rivets, crimping, silver soldering or other
well-known attachment techniques.
Referring to FIG. 5A, a cross-sectional view looking in the direction of
the arrows labelled 5A of the embodiment of FIG. 1 is shown. Rod 112 is
shown disposed within the float 136. Hole 138 is sized to receive and
center rod 112 with respect to contacts 140 and 142 yet allow free
movement of float 136 vertically with respect to rod 112. Contact tips
140a and 142a make physical contact with conductor bars 126 and conductive
strip 132, respectively. Resistive strip 122 is shown attached to rod 112
and bars 126.
FIG. 6A is a partial front elevational view of the resistive strip 122 and
conductor bars 126 of the sensor 110. Conductor bars 126 extend from
resistive strip 122 at an angle slighly different from the perpendicular
to prevent contact tips 140a and 142a from falling in the non-conductive
area between each of the bars 126. Conductor bars 126 are uniformly spaced
and arranged in parallel to provide electrical contact with resistive
strip 122.
Sensors 10 and 110 are three wire position sensors or transducers. Three
wire sensors are connectable to a fixed DC reference voltage (at terminals
44 and 46 or terminals 144 and 146) so that a reliable wiper voltage is
available (at terminals 48 or 148). Voltage sensing obviates resistance
deviations of the strips 22 and 122 which may occur as a result of
temperature, aging, or surface oxidation and erosion. Sensors 10 and 110
convert the physical position of floats 36 and 136, respectively, into a
resistance or voltage which is directly proportional to the liquid level
in the container. Terminals 44 and 46 are resistor connections, and
terminal 48 is the wiper connection of the potentiometric sensor 10.
Likewise, terminals 144 and 146 are resistor connections, and terminal 148
is the wiper connection of the potentiometric sensor 110. A suggested
application of the sensors 10 and 110 is hereinafter described. The
sensors 10 or 110 are vertically oriented in a fuel tank and secured
thereto by a mounting flange at the top and bottom of the sensors.
Terminal 44(144), terminal 46 (146), and terminal 48(148) are connectable
to outside circuitry to provide a resistive or proportional voltage signal
corresponding to liquid level in the fuel tank. The sensors 10 and 110 may
also be mounted within a baffle cylinder (not shown) which is normally
attached to a mounting flange. The baffle cylinder attenuates sloshing
about of the fuel in the tank when the motor vehicle is in motion. The
baffle cylinder enables stable and reliable measurements of the liquid
level.
Sensor 10 is economically practical in the 10 to 25 cm length range, which
covers the majority of automotive applications. For smaller length sensors
and lower quantity requirements the use of sensor 110 made of alumina
ceramic substrates is preferred. Sensors 110 in the 7 to 15 cm length can
be fabricated from laser scribed ceramic substrates very quickly with very
little tooling cost.
Sensors using thick film cermet on porcelain and ceramic substrates are
satisfactory for use in gasoline, diesel fuel, methanol blends,
lubricating oil, refrigerant and other non-conductive, relatively
uncontaminated fluids.
While the invention has been illustrated and described in detail in the
drawings and foregoing description, the same is to be considered as
illustrative and not restrictive in character, it being understood that
only the preferred embodiment has been shown and described and that all
changes and modifications that come within the spirit of the invention are
desired to be protected.
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